Seems like a pretty simple design to me, the only thing I would imagine would be tricky would be the 2nd level menu that pops up for menu items like Product details. You can configure multiple instances skin objects (like the DNNNav object) with different settings (scope, style, etc) however, in that site it changes the background image to match the new row being added. All together though, I wouldn't imagine that being that difficult for anyone with average HTML/CSS skills and some skill dropping in and styling DNN Tokens / Objects. 

Nina - when you say that you have used the Links SO to get it complaint, is that in the case of single-level menus only? The reason I ask is that several sites I have put out recently (based on 5.1.2) have passed W3C with 0 errors, and they use the DNNNav 2.0 Nav system.

To the OP - What Nina said about the difficulty factor is pretty spot-on. Most of this comes down to experience. If you are fairly comfortable using HTML and CSS, skinning for DNN can be picked up fairly easily. However, if you don't understand those concepts, you are basically having to learn both at the same time, which can lead to considerable frustration and confusion.

Good luck!